Redox indicator

A substance whose color changes with oxidation state, used to show the endpoint or progress of an oxidation–reduction reaction.

Redox potential: A measure of a chemical species’ tendency to gain or lose electrons under specified conditions. The indicator changes color when the solution potential crosses its characteristic transition range.
Redox titration: A titration in which the reaction between analyte and titrant involves electron transfer. A color-changing indicator can mark the titration endpoint.
Oxidation state: A formal number assigned to an atom to track electron accounting in compounds and ions. The indicator’s oxidized and reduced forms differ in oxidation state.
Ferroin: A redox indicator formed by an iron(II) complex with three 1,10-phenanthroline ligands. Its red iron(II) form becomes pale blue when oxidized to iron(III).
Nernst equation: An equation relating electrode potential to the activities of oxidized and reduced species. It describes how the indicator’s oxidized-to-reduced ratio shifts with potential.
Permanganate titration: A redox titration using permanganate ions as the oxidizing titrant. Permanganate’s intense color often makes a separate indicator unnecessary.
Electron transfer: The movement of an electron from one atom, molecule, or ion to another. Electron transfer converts the indicator between its differently colored forms.
Diphenylamine: An aromatic amine used as a redox indicator in some oxidation titrations. Its oxidation produces colored species that signal endpoints in dichromate titrations.
Oxidation–reduction reaction: A chemical reaction in which electrons move between species, changing their oxidation states. The indicator reports the progress of this broader class of electron-transfer reactions.
Cerimetry: A volumetric analysis method that uses cerium(IV) as an oxidizing titrant. Redox indicators can reveal the endpoint when cerium(IV) itself gives insufficient visual contrast.

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